Researchers in Canada say that Truvada, one of the most promising pre-exposure prophylaxis medications designed to prevent patients from contracting HIV, has failed a 43-year-old Canadian man.

The patient is the first recorded case study of a regular Truvada user contracting the disease, according to Poz, a major print and online publication focusing on communities affected by HIV. Researchers say that if taken diligently, at least four times a week, the drug provides 99% protection from the HIV virus for men who have sex with other men (MSM).

Unfortunately, in science there are very few absolutes. Although pre-exposure prophylaxis, or PrEP, is highly efficient in preventing the disease, it cannot claim a 100% success rate. Still, a 99% effectiveness rate is a monumental achievement in the fight against HIV and AIDS, which, when coupled with needle-exchange and other educational programs, is getting more effective by the day.

The biggest problem doctors and their patients face with PrEP treatments is adherence to a strict regimen of the drugs. The CDC recommends daily use of the drugs for maximum effectiveness, although researchers have found Truvada and other PrEP drugs have a certain amount of leeway, as long as patients are taking them at least four times a week. In clinical studies, many patients taking PrEP medications contracted HIV, though always after an extended lapse in taking their pills — the latest case study is the first observed case in a diligent user.

Despite the anomalous case study, published by a team of researchers at the Conference on Retroviruses and Opportunistic Infections in Boston, taking PrEP treatments regularly can be life-changing for men and transgender women at the highest risk of contracting HIV. While doctors don’t quite have a cure for the disease, they’re getting close, and preventative measures have curbed the epidemic in affluent countries, although it’s still a major crisis in developing nations.

Though viruses like Zika and Ebola are still major crises, researchers say we’ve made great strides against HIV with prevention drugs like Truvada and cost-effective treatments for those in greatest need.

Photos via Getty